[Pw_forum] About calculating exchange parameters

[Pang Rui]

Dear all:
I want to calculate the magnetic coupling constant J so that I can get the
Cruie temperature. How can I achieve this by using QE? 
In some of the articles, the exchange parameter J was calculated by the
energy difference of antiferromagnetism(AFM) state and ferromagnetism(FM)
state. However, my initial antiferromagnetic moments usually cannot remain
antiferromagnetic during the SCF process. Are these AFM(FM) states obtained
by constrained magnetization calculation? 
Thanks for the help.